A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
The dimensions (e.g. line width, or critical dimension) of pattern features that can be applied to a substrate are limited by the wavelength of radiation which forms a radiation beam that is used to provide those features on the substrate. In order to decrease the minimum feature size, it is therefore possible to use radiation of a shorter wavelength. In practice, however, it is often technically difficult and expensive to reduce the wavelength to, for example, wavelengths in the extreme ultraviolet range of the electromagnetic spectrum in order to decrease the minimum feature size. Therefore, in order to reduce the feature sizes that can be applied to a substrate, different approaches have been investigated. One approach to reducing the feature sizes of patterns applied to a substrate is double patterning. Double patterning is a broad term which covers many techniques which are used to provide pattern features on a substrate which are, for example, dimensioned or spaced apart by distances which could not be achieved by using a single exposure and single development of a resist patterned by that exposure.
One example of double patterning is known as double exposure. Double exposure is a sequence of two separate exposures of the same layer of resist using two different masks (or the same mask that has been shifted in order to shift the target location of a pattern to be applied to the resist). The substrate and/or mask can be moved distances which are far smaller than the wavelength of the radiation used to expose the resist. In one example, the resist can be exposed to provide a first pattern. The substrate and/or mask can then be moved and a second exposure undertaken to provide a second pattern, ensuring that features of the second pattern are located in-between (e.g. interdigitated with respect to) features of the first pattern. The first pattern and second pattern are both, independently, subject to the same limits which are imposed on the minimum pattern feature size by the wavelength of radiation used in each exposure. However, because the pattern features of the combined first and second patterns are located in-between one another (e.g. interdigitated) the pattern features may be closer together than would have been achievable using only a single exposure. One problem with this approach, however, is that the first and second exposures have to be accurately aligned to ensure that the spacing between the pattern features (or, in other words, the overlay) in the resulted combined pattern is as desired. This can be difficult to reliably and consistently achieve.
Another approach is sometimes referred to as a spacer lithography process, or a self-aligned spacer process (as well as many other variations thereon). This process involves providing a first pattern feature (or more than one first pattern feature) on a substrate. The minimum dimensions of this first pattern feature are, as described above, subjected to the limit imposed by the wavelength of radiation used to provide the pattern feature. Material is then provided on the first pattern feature, coating sidewalls of the first pattern feature. The coatings on the sidewalls are known as spacers, giving this approach its name. The first pattern feature itself is then removed, but the material that was on the sidewalls remains. This material forms two second pattern features which are separated by the width of the original first pattern feature. Thus, two second pattern features are formed in place of a single first pattern feature—the second pattern features have, for example, approximately half the pitch of the original first pattern feature. The pitch is halved without having to decrease the wavelength of radiation used.
In a spacer lithography process, only a single exposure is undertaken, and so there is no need to consider the alignment or overlay requirements associated with the double exposure process discussed above. A different problem is, however, encountered in the spacer lithography process. For instance, it is desirable to ensure that the second pattern features have the same dimensions as each other (e.g. the same line width), and that the second pattern features are equally spaced with respect to one another, to ensure that a pattern provided on a substrate is as regular and uniform as possible. This is difficult to achieve.